My prediction is: Yes.
What that means that the BH is a spherical symmetrical object and that
there exists a 'gaseous' layer outside the radius of the BH which emits
light in all? directions.
[Moderator's note: Almost all, or all, astrophysical black holes are not spherically symmetric in the sense that they rotate.
Rotating black holes are more complicated. What an observer actually
sees when looking at a black hole is not trivial to calculate.
In any case, the general
consensus is that black holes detectable via radiation emitted from near
them have an accretion disk and thus aren't spherically symmetric.
Op vrijdag 24 juni 2022 om 12:07:34 UTC+2 schreef Nicolaas Vroom:
What that means that the BH is a spherical symmetrical object and that
there exists a 'gaseous' layer outside the radius of the BH which emits
light in all? directions.
SNIP
[Moderator's note: Almost all, or all, astrophysical black holes are not
spherically symmetric in the sense that they rotate.
My understanding is that also all stars and planets rotate, at the same
time many of these could be called spherical symmetric like our earth
and the Sun.
Rotating black holes are more complicated. What an observer actually
sees when looking at a black hole is not trivial to calculate.
The first step is to observe. To calculate is a second step.
In any case, the general
consensus is that black holes detectable via radiation emitted from near
them have an accretion disk and thus aren't spherically symmetric.
The question is to what extend can we conclude, based on observations,
that the BH, part of Sagittarius A*, has an accretion disk. Observing
the picture in https://www.nature.com/articles/d41586-022-01320-y the
ring surrounding the BH must be an 'indication' of this disk. If that is
the case the ring must be situated in a plane almost perpendicular
towards the direction of the line of sight between the earth and the
centre of the BH. (1) This direction must also be the same as the axis
of rotation of the BH.
Assuming that the ring is part of an accretion disk, I should expect,
that if we travel around this BH, like the Sun does around the BH, the
shape of this ring, as observed from our spaceship, must also change.
This shape must be almost the same after we have travelled 180 degrees
and the same after 360 degrees.
As I mentioned before, I have my doubts. The ring does not change and
there is no prove of an accretion disk, based on this image.
What also is in favour of a sperical object is that the movement of the
stars around the BH is random. There is no preference.
I found also a different article: https://www.nasa.gov/feature/goddard/2021/hubble-mini-jet-found-near-milky-ways-supermassive-black-hole
This article also shows the direction of rotation of the BH.
The direction is different compared with (1) above.
My impression is that when you read this article and other articles the accretion disks are of temporary nature and depend about source, that
causes the inflow of material. Together the BH and the source can be considered as a binary system.
As mentioned above the movement of the stars around Sagitarrius A* are random, as such, my guess is, that the direction of possible accretion
disks is also random, which is in contradiction with observation (1)
Op vrijdag 24 juni 2022 om 12:07:34 UTC+2 schreef Nicolaas Vroom:
[[Mod. note -- A few comments:
1. The Earth is *approximately* spherically symmetric, but if you look
more closely its shape is in fact rotationally flattened. That is,
the Earth's equatorial radius is about 0.34% larger than its polar
radius, so the Earth is in fact NOT spherically symmetric.
2. While it's true that if we travel around the Sgr A* BH, its apparent
shape will change, that doesn't help us right now: our solar system
takes around 250 million years to orbit the centre of our galaxy,
so we're not going to get to look at the Sgr A* BH from a
significantly different orientation any time in our lives.
3. Accretion disks (including the one around the Sgr A* BH) are indeed temporary and depend on the availability of source matter. But I
wouldn't say that the BH and the source are a "binary" system, because there's no reason to think that the source is a single compact object. Rather, the BH is embedded in a cloud of (moving) stars and
interstellar gas.
4. This 2020 article by Fragione & Loeb, https://iopscience.iop.org/article/10.3847/2041-8213/abb9b4
(which argues for a relatively low (slow) spin for the Sgr A* BH)
notes that past studies have given conflicting values for that spin.
I don't know enough about this subject to have an informed opinion
myself. Given the instruments now operational, we should know a
*lot* more about this in a few years, especially once ESO's
Extremely Large Telescope is operational (planned for 2027ish).
-- jt]]
On 28/06/2022 08:28, Nicolaas Vroom wrote:
The question is to what extend can we conclude, based on observations,It would be incredibly surprising if it did not have an accretion disk
that the BH, part of Sagittarius A*, has an accretion disk. Observing
the picture in https://www.nature.com/articles/d41586-022-01320-y the
ring surrounding the BH must be an 'indication' of this disk. If that is the case the ring must be situated in a plane almost perpendicular
towards the direction of the line of sight between the earth and the
centre of the BH. (1) This direction must also be the same as the axis
of rotation of the BH.
of some sort if there is any matter near enough to be subject to being
pulled in. It has to lose angular momentum somehow to fall into it.
The black hole has strong enough gravity to bend light paths over the
poles so that you see something that is quite distorted from whatever
angle you look. Raytracers have simulated this. I am surprised how close
to a blurred version of their predictions the observations have been!
Assuming that the ring is part of an accretion disk, I should expect,
that if we travel around this BH, like the Sun does around the BH, the shape of this ring, as observed from our spaceship, must also change.
This shape must be almost the same after we have travelled 180 degrees
and the same after 360 degrees.
Not if the thing is interacting with matter. Bright spots on the
accretion disk may change on timescales worryingly close to the time
required to obtain enough data for a satisfactory image of the target.
[[Mod. note -- You're mistaken. The light we is is that which
*eventually* is pointing in our direction, but it may have been
emitted in a very different direction (and then had its path bent by
the strong gravitational field into one pointing in our direction).
-- jt]]
Op donderdag 7 juli 2022 om 08:11:24 UTC+2 schreef Martin Brown:
On 28/06/2022 08:28, Nicolaas Vroom wrote:
The black hole has strong enough gravity to bend light paths over the
poles so that you see something that is quite distorted from whatever
angle you look. Raytracers have simulated this. I am surprised how close
to a blurred version of their predictions the observations have been!
That is correct.
But this light can come from all directions and also be emitted in all directions. Part of that emitted light can come in our direction.
Assuming that the ring is part of an accretion disk, I should expect,
that if we travel around this BH, like the Sun does around the BH, the
shape of this ring, as observed from our spaceship, must also change.
This shape must be almost the same after we have travelled 180 degrees
and the same after 360 degrees.
Not if the thing is interacting with matter. Bright spots on the
accretion disk may change on timescales worryingly close to the time
required to obtain enough data for a satisfactory image of the target.
My assumption is that this accretion disc is more or less fixed to the
BH and lies in the plane of the picture. That means if you travel in 80 days
around this BH that when you return after 80 days the picture should
be more or less the same. But the intermediate pictures should not.
If you start from a circle after 10 days this should be an ellipse after
20 days a vertical beam, after 30 days an ellipse and after 40 days again
a circle. What I mean is that to observe a circle is rare.
At the same time, that is my guess, if the BH would be surrounded, with
a more or less equally distributed layer of some gaseous material, it
is possible that you always observe this more or less doughnut shaped
visible ring.
[[Mod. note -- You're mistaken. The light we is is that which
*eventually* is pointing in our direction, but it may have been
emitted in a very different direction (and then had its path bent by
the strong gravitational field into one pointing in our direction).
-- jt]]
Is this not a philosophic point? What is more "our direction" than
the null geodesic which connects the light source to us? How do we
define the "straightness" which is *more* in "our direction"?
Mach's Principle gets in here, where it holds that there can be no
space wihout some matter to occupy it, i.e., matter is an inherent
part of any complete spatial manifold. Given that, it's pretty hard
to define something "straighter" than the null geodesic contoured by
the essential matter.
[[Mod. note --
1. That's not really what Mach's principle says. Among other things,
a "complete" spatial manifold (which implies that it doesn't contain
any black holes) may be a vacuum (contain no matter), but still
contain spacetime curvature (nonzero Riemann tensor), e.g.,
gravitational waves and/or geons
https://en.wikipedia.org/wiki/Geon_(physics)
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